Martin Guptill hit a four off the last ball to score a century and guide New Zealand to a Twenty20 international cricket victory over South Africa in East London this morning (NZ time).

Guptill finished unbeaten on 101 as the Black Caps scored 169-2, chasing down the big total off just 19 overs in a match reduced after a floodlight failed early in the South African evening.

After being comprehensively beaten by eight wickets in the series-opener in Durban on Saturday, the Black Caps finally found their form to win the second match in the most dramatic fashion.

And there was no bigger hero than Guptill. He missed the Durban match with a stomach upset but got off his sick-bed to dominate at Buffalo Park.

Guptill became just the second New Zealand batsman - after skipper Brendon McCullum - to score a T20 century, finishing on 101, scored off just 69 balls with nine fours and six sixes.

His last boundary was the sweetest, raising his century and bringing the much-needed victory to a team under plenty of pressure.

New Zealand needed 11 off the last over bowled by Rory Kleinveldt and edged their way into a position where it all came down to Guptill on the last ball. He delivered in spectacular fashion to level the three-match series.

The final and deciding match in the series will be played in Port Elizabeth on Thursday morning (NZ time).

New Zealand had been set the difficult task of scoring 169 off 19 overs in a total readjusted under the Duckworth-Lewis system after the power failure sent the players form the field for 45 minutes during South Africa’s innings.

From 128-2 after 16.1 overs at the unexpected break, the Proteas plundered 37 runs off the last 17 balls when play finally resumed. South Africa finished at 165-5 off their 19 overs but got a few extra runs under the controversial adjustment system.

That left the Black Caps needing to score more than nine runs an over for victory and at times that got up beyond 10.

But they kept their cool, mixing aggressive boundary shots with some good running between the wickets.

Guptill and Rob Nicol put on 76 for the opening wicket - a record against South Africa - before Nicol holed out, scoring 25 off 24 balls.

Guptill kept going and got good support from McCullum (17 off 15) and Colin Munro (eight off seven) to see the Kiwis home to a cherished victory.

Guptill had a bit of luck towards the end of the innings. On 89, he was caught off a Morne Morkell delivery that was subsequently ruled a no-ball because it was higher than the batsman's waist. He was also dropped on 89 as South Africa's fielders started to wilt under the growing pressure.

Earlier New Zealand had won the toss and decided to bowl.

They claimed the early wicket of Richard Levi but then got punished by Henry Davids who scored 55 from 38 balls before being brilliantly caught by Jimmy Neesham in the deep.